1. Preheat the oven to 175°C (350°F).
2. Separate the egg whites from the yolks. In a stand mixer, beat the egg yolks on high speed with ¼ of the sugar until thick and pale (= ribbon stage), about 5 to 8 minutes.
3. Melt the chocolate and butter in the microwave. Then, incorporate the salt and beaten egg yolks, in 3 additions, using a spatula.
4. Whip the egg whites with the remaining sugar to make a French meringue.
5. Mix a little bit of the meringue into the chocolate batter. Then, incorporate the rest of the meringue, in 3 additions, into the chocolate batter. Use a spatula and keep the mixture as airy as possible.
6. Line the mold with parchment paper. Pour in ½ of the batter. Smooth the surface and bake the cake for 20 to 22 minutes in the preheated oven.
7. Cover the other half of the batter with plastic wrap to prevent it from drying out. Let it rest on the countertop, in a warm place.
8. Remove the cake from the oven and let it cool for 20 minutes in the mold. Then, pour the other half of the batter on top and smooth the surface with a palette knife. Let it cool to room temperature, then store in the refrigerator for 1 to 3 days. You can also store the cake in the freezer.
9. Place the cake in the freezer for 30 minutes to make it easier to unmold (or remove it from the freezer if necessary and let it thaw for 20 to 30 minutes). Gently unmold and generously sprinkle with cocoa powder.